Method for determining if a fuel cell stack is overheating using stack end plate temperature sensors

ABSTRACT

A method for determining whether a fuel cell stack is overheating. The method measures the temperature of end cells in the stack using end cell temperature sensors, and calculates an average end cell temperature based on the end cell temperature measurements. The method also measures the temperature of a cooling fluid being output from the fuel cell stack. The method determines if any of the measured end cell temperatures are outlying by comparing each end cell temperature measurement to the average. The method determines that the cooling fluid outlet temperature sensor has possibly failed if the cooling fluid outlet temperature is greater than the average end cell temperature and the cooling fluid outlet temperature minus the average end cell temperature is greater than a predetermined temperature value.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional application of U.S. patent applicationSer. No. 11/640,087, filed Dec. 15, 2006, titled “Fuel Cell ReliabilityImprovement by Using Stack End Plate Temperature Sensors.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a method for determining whether afuel cell stack is overheating and, more particularly, to a method fordetermining whether a fuel cell stack is overheating using end celltemperature sensors.

2. Discussion of the Related Art

Hydrogen is a very attractive fuel because it is clean and can be usedto efficiently produce electricity in a fuel cell. A hydrogen fuel cellis an electro-chemical device that includes an anode and a cathode withan electrolyte therebetween. The anode receives hydrogen gas and thecathode receives oxygen or air. The hydrogen gas is dissociated in theanode to generate free protons and electrons. The protons pass throughthe electrolyte to the cathode. The protons react with the oxygen andthe electrons in the cathode to generate water. The electrons from theanode cannot pass through the electrolyte, and thus are directed througha load to perform work before being sent to the cathode.

Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell forvehicles. The PEMFC generally includes a solid polymer electrolyteproton conducting membrane, such as a perfluorosulfonic acid membrane.The anode and cathode typically include finely divided catalyticparticles, usually platinum (Pt), supported on carbon particles andmixed with an ionomer. The catalytic mixture is deposited on opposingsides of the membrane. The combination of the anode catalytic mixture,the cathode catalytic mixture and the membrane define a membraneelectrode assembly (MEA).

Several fuel cells are typically combined in a fuel cell stack togenerate the desired power. The fuel cell stack receives a cathodereactant gas, typically a flow of air forced through the stack by acompressor. Not all of the oxygen is consumed by the stack and some ofthe air is output as a cathode exhaust gas that may include water as astack by-product. The fuel cell stack also receives an anode hydrogenreactant gas that flows into the anode side of the stack. The stack alsoincludes flow channels through which a cooling fluid flows.

The fuel cell stack includes a series of bipolar plates positionedbetween the several MEAs in the stack, where the bipolar plates and theMEAs are positioned between two end plates. The bipolar plates includean anode side and a cathode side for adjacent fuel cells in the stack.Anode gas flow channels are provided on the anode side of the bipolarplates that allow the anode reactant gas to flow to the respective MEA.Cathode gas flow channels are provided on the cathode side of thebipolar plates that allow the cathode reactant gas to flow to therespective MEA. One end plate includes anode gas flow channels, and theother end plate includes cathode gas flow channels. The bipolar platesand end plates are made of a conductive material, such as stainlesssteel or a conductive composite. The end plates conduct the electricitygenerated by the fuel cells out of the stack. The bipolar plates alsoinclude flow channels through which a cooling fluid flows.

For automotive applications, it typically takes about 400 fuel cells toprovide the desired power. Because so many fuel cells are required forthe stack in automotive fuel cell system designs, the stack is sometimessplit into two sub-stacks each including about 200 fuel cells because itis difficult to effectively provide an equal flow of hydrogen gasthrough 400 fuel cells in parallel.

A fuel cell system typically includes a thermal sub-system for coolingthe fuel cell stack to a desired operating temperature. The thermalsub-system includes a pump that pumps a cooling fluid through a coolantloop outside of the stack and cooling fluid flow channels providedwithin the bipolar plates. A radiator typically cools the hot coolingfluid that exits the stack before it is sent back to the stack.

The end cells in a fuel cell stack typically have a lower performancethan the other cells in the stack. Particularly, the end cells areexposed to ambient temperature, and thus have a temperature gradientthat causes them to operate at a lower temperature as a result ofconvective heat losses. Because the end cells are typically cooler thanthe rest of the cells in the stack, gaseous water more easily condensesinto liquid water so that the end cells have a higher relative humidity,which causes water droplets to more readily form in the flow channels ofthe end cells. Further, at low stack load, the amount of reactant gasflow available to push the water out of the flow channels issignificantly reduced. Also, at low stack loads the temperature of thecooling fluid is reduced, which reduces the temperature of the stack andtypically increases the relative humidity of the reactant gas flow.

It is known in the art to heat the end cells with resistive heaterspositioned between a unipolar plate and an MEA so as to compensate forconvective heat losses. These systems typically attempted to maintainthe end cell temperature the same as the other cells in the stack bymonitoring the temperature of the cooling fluid out of the stack.However, lower cell voltages for the end cells may still be a problemeven with the addition of such heaters.

Various components in the fuel cell stack, such as the membranes, may bedamaged if the temperature of the stack increases above a certainmaterials transition temperature, such as 85° C. Therefore, fuel cellsystems typically employ a cooling fluid temperature monitoringsub-system that monitors the temperature of the cooling fluid flowingout of the stack so as to prevent the temperature of the stack fromincreasing above a predetermined temperature. Various factors couldcause the temperature of the fuel stack to increase above thepredetermined temperature, such as operating the stack at a high loadfor an extended period of time in a high ambient temperatureenvironment.

In current fuel cell system designs, the cooling fluid temperature istypically measured at the cooling fluid outlet from the stack by atemperature sensor. If the cooling fluid is flowing, the sensor wouldprovide an indication of stack overheating. If the cooling fluid, andthus the fuel cell stack, becomes overheated, the system may be shutdown to protect the stack. However, there are potential failure modeswhere the system might not detect stack overheating, or detect a falseoverheating condition causing an unnecessary system shut down. Thesepotential failure modes include cooling fluid pump failure, coolantfluid loss, cooling fluid flow blockage and cooling fluid outlettemperature sensor failure. If the system does not detect an overheatcondition of the fuel cell stack, the stack membranes may becomedamaged. However, if the system falsely detects an overheating conditionand shuts the system down, system reliability will be lower.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, a method isdisclosed for determining whether a fuel cell stack is overheating. Themethod measures the temperature of end cells in the stack using endplate or end cell temperature sensors, and calculates an average endcell temperature based on the end cell temperature measurements. Themethod also measures the temperature of a cooling fluid being outputfrom the fuel cell stack. The method determines if any of the measuredend cell temperatures are outlying by comparing each end celltemperature measurement to the average. The method determines that thecooling fluid outlet temperature sensor has possibly failed if thecooling fluid outlet temperature is greater than the average end celltemperature and the cooling fluid outlet temperature minus the averageend cell temperature is greater than a predetermined temperature value.The method also determines that the fuel cell stack may be overheatingif the average end cell temperature is greater than the cooling fluidoutlet temperature and the average end cell temperature minus thecooling fluid outlet temperature is greater than a predeterminedtemperature value.

Additional features of the present invention will become apparent fromthe following description and appended claims, taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a fuel cell system includingsplit stacks having end cell temperature sensors; and

FIG. 2 is a flow chart diagram showing a process for detecting stackoverheating using the end cell temperature sensors, according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the invention directed toa method for determining fuel cell stack overheating by using endtemperature sensors is merely exemplary in nature, and is in no wayintended to limit the invention or its applications or uses.

FIG. 1 is a schematic block diagram of a fuel cell system 10 including afirst split fuel cell stack 12 and a second split fuel cell stack 14.The split stack 12 receives a cathode input airflow on line 16 and thesplit stack 14 receives a cathode input airflow on line 18, typicallyfrom a compressor (not shown). Cathode exhaust gas is output on line 20from both of the split stacks 12 and 14. The split stacks 12 and 14employ anode flow shifting where the anode reactant gas flows back andforth through the split stack cells 12 and 14 at a predetermined cycle.Therefore, the anode reactant gas flows in and out of the split stack 12on line 22 and in and out of the split stack 14 on line 24. An anodeconnecting line 26 connects the anode channels in the split stacks 12and 14.

The split stack 12 includes end cell heaters 28 and 30 positioned withinend cells 32 and 34, respectively, of the split stack 12. Likewise, thesplit stack 14 includes end cell heaters 36 and 38 positioned within endcells 40 and 42, respectively, of the split stack 14. The end cellheaters 28, 30, 36 and 38 are positioned at a suitable location in theend cells of the split stacks 12 and 14, such as between the unipolarplate and the adjacent MEA. The end cell heaters 28, 30, 36 and 38 canbe any heater suitable for the purposes described herein, such asresistive heaters, well understood to those skilled in the art.

According to the invention, an end plate or end cell temperature sensor44 is provided in the end cell 32 and an end plate or end celltemperature sensor 46 is provided in the end cell 34 of the split stack12. Likewise, an end plate or end cell temperature sensor 48 is providedin the end cell 40 and an end plate or end cell temperature sensor 50 isprovided in the end cell 42 of the split stack 14. The temperaturesensors 44, 46, 48 and 50 can be any temperature sensor suitable for thepurposes discussed herein, such as thermocouples.

A pump 52 pumps a cooling fluid through a coolant loop 54 and throughcooling fluid flow channels in the split stacks 12 and 14 to control thestack operating temperature, as is well understood in the art. Atemperature sensor 56 is provided in the coolant loop 54 at an outputfrom the split stack 12. A controller 58 controls the end cell heaters28, 30, 36 and 38 and the pump 52, and receives temperature measurementreadings from the temperature sensors 44, 46, 48, 50 and 56 inaccordance with an algorithm of the present invention, as will bediscussed in more detail below.

According to the invention, the end plate temperature sensors 44, 46, 48and 50 detect whether the split stacks 12 and 14 are overheating.Because the end plate temperature sensors 44, 46, 48 and 50 are closerto the reaction site of the stacks 12 and 14, as compared to the stackcooling fluid outlet temperature sensor 56, stack overheating can bedetected faster before membrane damage may occur. Further, unlike thestack cooling fluid outlet temperature sensor 56, the end platetemperature sensors 44, 46, 48 and 50 can detect overheating even whenthe cooling fluid is not flowing through the stacks 12 and 14. Thepresent invention also uses the end plate temperature sensors 44, 46, 48and 50 to detect stack coolant outlet temperature sensor failure. If astack cooling fluid outlet temperature sensor failure occurs, theaverage end plate temperature calculations are used, and any unnecessarysystem shut down is avoided.

FIG. 2 is a flow chart diagram 80 showing an algorithm for determiningwhether the split stacks 12 and 14 are overheating using the end platetemperature sensors 44, 46, 48 and 50, according to an embodiment of thepresent invention. Although the algorithm is discussed with reference toa split stack design, the algorithm is also applicable to be used in asingle stack, where only two end plate temperature sensors would beemployed, one in each end cell. The algorithm first disables the endcell heaters 28, 30, 36 and 38 at box 82. The algorithm then measuresthe temperature of the end cells 32, 34, 40 and 42 using the end platetemperature sensors 44, 46, 48 and 50 at box 84. Based on the fourtemperature measurements, the algorithm calculates an average end celltemperature T_(ec,avg) at box 86. The algorithm then subtracts theaverage temperature T_(ec,avg) from each individual measured temperatureT_(eci) from the end cell temperature sensors 44, 46, 48 and 50, anddetermines whether the absolute value of the subtracted value is lessthan a predetermined temperature value, for example, 5° C. at decisiondiamond 88. If any of the individual measured temperature readings areoutside of this threshold, they are excluded as outlying temperaturemeasurements at box 90 to ensure that the average calculation has asmall standard of deviation. The algorithm then proceeds back to the box86 to calculate the average temperature without the temperature readingsthat were outside of the threshold.

When all of the measured temperature readings from the temperaturesensors 44, 46, 48 and 50 are within the threshold at the decisiondiamond 88, the algorithm determines whether the cooling fluid outlettemperature T_(stack) _(—) _(out) is greater than the calculated averagetemperature T_(ec,avg) and subtracts the calculated average temperatureT_(ec,avg) from the cooling fluid outlet temperature T_(stack) _(—)_(out) at decision diamond 92. If the cooling fluid outlet temperatureT_(stack) _(—) _(out) is greater than the calculated average temperatureT_(ec,avg) and the cooling fluid outlet temperature T_(stack) _(—)_(out) minus the average temperature T_(ec,avg) is greater than apredetermined temperature value, for example, 3° C., then the algorithmdetermines that the cooling fluid temperature sensor 56 may have failedat box 94. In this situation, the algorithm will keep the end cellheaters 28, 30, 36 and 38 disabled, maintain the calculated averagetemperature T_(ec,avg) as a control feedback, keep the split stacks 12and 14 running and report the occurrence as a diagnostic.

If the cooling fluid outlet temperature T_(stack) _(—) _(out) is notgreater than the calculated average temperature T_(ec,avg) and/or thecooling fluid outlet temperature measurement T_(stack) _(—) _(out) minusthe calculated average temperature measurement T_(ec,avg) is 3° C. orless at the decision diamond 92, the algorithm then determines whetherthe calculated average temperature T_(ec,avg) is greater than thecooling fluid outlet temperature measurement T_(stack) _(—) _(out) anddetermines whether the calculated average temperature T_(ec,avg) minusthe cooling fluid outlet temperature measurement T_(stack) _(—) _(out)is greater than a predetermined temperature value, for example, 5° C.,at decision diamond 96. If the calculated average temperature T_(ec,avg)is greater than the cooling fluid outlet temperature measurementT_(stack) _(—) _(out) and the calculated average temperature T_(ec,avg)minus the cooling fluid outlet temperature T_(stack) _(—) _(out) isgreater than the predetermined value, then the algorithm determines thata stack overheating condition may be occurring, from, for example, pumpfailure, cooling fluid loss in the system, or possible cooling fluidoutlet temperature sensor failure at box 98. The algorithm then willreport the incident to the vehicle diagnostics system, and shut the fuelcell stack down. If the calculated average temperature T_(ec,avg) isless than the cooling fluid outlet temperature measurement T_(stack)_(—) _(out) and/or the calculated average temperature T_(ec,avg) minusthe cooling fluid outlet temperature T_(stack) _(—) _(out) is less than5° C., then the algorithm determines that the stacks 12 and 14 are notoverheating, and continues with a normal run mode at box 100.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion and from the accompanyingdrawings and claims that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

1. A method for determining whether a fuel cell stack is overheating,said method comprising: measuring the temperature of end cells in thefuel cell stack; measuring the temperature of a cooling fluid beingoutput from the fuel cell stack using a cooling fluid outlet temperaturesensor; and determining if the fuel cell stack may be overheating basedon the temperature of the cooling fluid and the temperature of the endcells, wherein determining that the fuel cell stack may be overheatingincludes determining if an average end cell temperature is greater thanthe measured cooling fluid outlet temperature and the average end celltemperature minus the measured cooling fluid outlet temperature isgreater than a predetermined temperature value.
 2. The method accordingto claim 1 wherein determining if the fuel cell stack may be overheatingincludes determining that the fuel cell stack may be overheating if theaverage end cell temperature minus the measured cooling fluid outlettemperature is greater than 5° C.
 3. The method according to claim 1further comprising disabling end cell heaters prior to using thetemperature of the end cells.
 4. The method according to claim 1 whereinthe fuel cell stack is a split stack, and wherein measuring thetemperature of end cells in the fuel cell stack includes measuring fourtemperatures including two temperatures from the end cells of one splitstack and two temperatures of the end cells of another split stack.
 5. Amethod for determining whether a fuel cell stack is overheating, saidmethod comprising: measuring the temperature of end cells in the fuelcell stack; measuring the temperature of a cooling fluid being outputfrom the fuel cell stack using a cooling fluid outlet temperaturesensor; and determining if the fuel cell stack may be overheating basedon the temperature of the cooling fluid and the temperature of the endcells, wherein determining if the fuel cell stack may be overheatingincludes calculating an average end cell temperature from thetemperature of the end cells, determining if any of the end celltemperatures are outlying by comparing each end cell temperature to theaverage end cell temperature, and recalculating the average end celltemperature without the outlying temperature measurements, and whereindetermining if the fuel cell stack may be overheating further includesdetermining that a measured end cell temperature is outlying if thedifference between the end cell temperature and average end celltemperature is greater than 5° C.
 6. The method according to claim 5further comprising disabling end cell heaters prior to using thetemperature of the end cells.
 7. The method according to claim 5 whereinthe fuel cell stack is a split stack, and wherein measuring thetemperature of end cells in the fuel cell stack includes measuring fourtemperatures including two temperatures from the end cells of one splitstack and two temperatures of the end cells of another split stack.
 8. Amethod for determining whether a fuel cell stack is overheating, saidmethod comprising: measuring the temperature of end cells in the fuelcell stack; measuring the temperature of a cooling fluid being outputfrom the fuel cell stack using a cooling fluid outlet temperaturesensor; and determining if the fuel cell stack may be overheating basedon the temperature of the cooling fluid and the temperature of the endcells, wherein determining if the fuel cell stack may be overheatingincludes determining that the cooling fluid outlet temperature sensorhas possibly failed if the measured cooling fluid outlet temperature isgreater than an average end cell temperature and the measured coolingfluid outlet temperature minus the average end cell temperature isgreater than a predetermined temperature value.
 9. The method accordingto claim 8 wherein determining if the fuel cell stack may be overheatingincludes determining that the cooling fluid outlet temperature sensorhas possibly failed if the cooling fluid outlet temperature minus theaverage end cell temperature is greater than 3° C.
 10. The methodaccording to claim 8 further comprising disabling end cell heaters priorto using the temperature of the end cells.
 11. The method according toclaim 8 wherein the fuel cell stack is a split stack, and whereinmeasuring the temperature of end cells in the fuel cell stack includesmeasuring four temperatures including two temperatures from the endcells of one split stack and two temperatures of the end cells ofanother split stack.
 12. A method for determining whether a fuel cellstack is overheating, said method comprising: measuring the temperatureof a cooling fluid being output from the fuel cell stack using a coolingfluid outlet temperature sensor; measuring the temperature of end cellsin the fuel cell stack; calculating an average end cell temperaturebased on the end cell temperature measurement; determining if any of themeasured end cell temperatures are outlying temperature measurements bycomparing each end cell temperature measurement to the average end celltemperature measurement; recalculating the average end cell temperaturewithout the outlying end cell temperature measurements; comparing themeasured cooling fluid outlet temperature to the average end celltemperature measurement; determining that the cooling fluid outlettemperature sensor has possibly failed if the measured cooling fluidoutlet temperature is greater than the average end cell temperaturemeasurement and the measured cooling fluid outlet temperature minus theaverage end cell temperature measurement is greater than a firstpredetermined temperature value; and determining that the fuel cellstack may be overheating if the average end cell temperature measurementis greater than the measured cooling fluid outlet temperature and theaverage end cell temperature measurement minus the measured coolingfluid outlet temperature is greater than a second predeterminedtemperature value.
 13. The method according to claim 12 wherein the fuelcell stack is a split stack, and wherein measuring the temperature ofend cells in the fuel cell stack includes measuring four temperaturesincluding two temperatures from the end cells of one split stack and twotemperatures of the end cells of another split stack.
 14. The methodaccording to claim 12 further comprising disabling end cell heaters inthe end cells of the fuel cell stack prior to measuring the temperatureof the end cells.
 15. The method according to claim 12 whereindetermining if any of the measured end cell temperatures are outlyingincludes determining that an end cell temperature measurement isoutlying if the difference between the end cell temperature measurementand the average end cell temperature measurement is greater than 5° C.16. The method according to claim 12 wherein determining that thecooling fluid outlet temperature sensor has possibly failed includesdetermining that the cooling fluid outlet temperature sensor haspossibly failed if the cooling fluid outlet temperature minus theaverage end cell temperature measurement is greater than 3° C.
 17. Themethod according to claim 12 wherein determining if the fuel cell stackmay be overheating includes determining that the fuel cell stack may beoverheating if the average end cell temperature measurement minus themeasured cooling fluid outlet temperature is greater than 5° C.